<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f1()">
<input type="button" value="减" onclick="f2()">
<input type="button" value="乘" onclick="f3()">
<input type="button" value="除" onclick="f4()">
<div>结果:</div>
<script>
  let d = document.querySelector("div");
  let i1 = document.querySelector("#i1");
  let i2 = document.querySelector("#i2");

  function f1() {
    if(isNaN(i1.value)||isNaN(i2.value)){
      d.innerText="输入错误!";
      return;
    }
    d.innerText="结果:"+(i1.value*1+i2.value*1);
  }
  function f2() {
    if(isNaN(i1.value)||isNaN(i2.value)){
      d.innerText="输入错误!";
      return;
    }
    d.innerText="结果:"+(i1.value-i2.value);
  }
  function f3() {
    if(isNaN(i1.value)||isNaN(i2.value)){
      d.innerText="输入错误!";
      return;
    }
    d.innerText="结果:"+(i1.value*i2.value);
  }
  function f4() {
    if(isNaN(i1.value)||isNaN(i2.value)){
      d.innerText="输入错误!";
      return;
    }
    d.innerText="结果:"+(i1.value/i2.value);
  }

</script>

</body>
</html>